I would like to use a good piston I have, in a project I have going but... The piston is too long. I need to shorten it to clear the flywheels. I have modified & shortened pistons before but not as much as this needs. What do you think? I could reduce the flywheel dia. but it's already been reduced some; not sure if reducing any further is practicable or desirable...?
Yes, slightly my feeling. Ideally I would like to try it but the stroke is long & the rod ratio is only 1.56 - I think the forces on the piston will be quite high. I can fit another shorter forged piston I have (robbing another project) which might last longer. It will need a comp. plate - easy enough - I haven't got dozens of pistons to play with. Some cars have run much lower rod ratios - 1.45 etc. - I think in low revving but big cc's motors. My understanding of long stroke & short rod & low rod ratio is that the piston is made to rock more - increasing ring wear / blow-by / oil consumption and the forces on the piston around TDC are more extreme. BSA did experiment with something similar - ending in broken pistons. Although they were trying to prove a point regarding long stroke vs short(er) stroke power. Long term plan is to race this but first need to get it assembled & running... This happens to be an "off the shed shelf" rod/crank & piston combo that all fit the barrel I have The barrel is not easily replaceable. There are a couple of other options but ending up with only a minor gain from std. All the other possibilities need either a custom rod, custom piston or custom crank - or combinations of them, so not cheap. I'd love to make a crank - I have a knackered lathe - but I think it's beyond my technical capabilities & equipment! If I could get hold of an early M20 crank (105 stroke) I'd be well chuffed but they were only made for a year or so. Can't be many surviving since 1937!
Not knowing what motor your actually running I’m guessing it’s a B33? The M series cranks were 4 bearing as opposed to three so you would need cases to fit (citation required) and to be honest I’m not sure what the standard rod:stroke ration is for that bike either but yes you would have a lot of piston rock.
One of my A65 builds (racer) is having A70 pistons and A10 rods fitted. The rods are half an inch longer than the A65 rods so rod ratio goes from 2:1 to about 2.2:1 with the primary reason to reduce piston rock and give less stress at higher rpms. I’m told that the shorter stroke will improve torque, much like a Manx norton but to what wear detriment?
Leg out of bed is always possible, even with good cranks (usually something else breaks first!) My 1st 2 race bikes were taper pin jobs that lasted 4 & 10 years. the 4 year was sold & the new owner over-revved it to destruction - leg out - although it was the rod that broke I believe, luckily most of the motor was rescuable. The 10 year motor big end finally went after many seasons hard use. I still have that crank although the shafts are loose - a common BSA pre-unit single thrashers problem The project is somewhat B33-ish ... The longer M series timing shaft can be machined down to B type dimensions, it's not too complicated. It is not perfect but will prove a point before making a proper crank, or just riding it around as a fun bike. Long stroke, big bore & low revs but with some grunt. Might pull high gears race wise. Certainly against big bore Rudges & OHC Nortons unlikely to win anything but will be a lark. I think the problem will be sorting the cams & timing, The ports are too small so I'm hoping the increased piston speed (I think) wiill assist in getting enough mixture in. Although this isn't one, actual B33's have been successfully raced against Gold Stars - but usually with lunatics, I mean bloody good riders on them... Currently I'm pursuing another crank possibility & mulling it all over. Suitable pistons are an issue, I could run a longer rod but the pin to piston deck distance has to be shorter than I've managed to find - with some kind of dome on it. Modern pistons are too flat
Last edited by flowboy; 12/11/1711:56 pm. Reason: more info
Have you read the Rolland Pyke memoirs on the gold stars. Certainly a valuable read before you have too much work done.
Port size is relative to how efficient it is in top gear, get that right and everything else follows, my road A65 has a small port head but big carbs. Plenty of torque and better top speed than the later big port head.
One head I modified I made with really small ports, it revved higher than any other head I had modified, and that was with air filters and small carbs. Unfortunately due to other problems it wouldn’t go over 90mph, re-modified the head and eventually found the real reason... never got to try the head again but it worked well.
Yes the memoirs were very interesting to a Goldie nut as I was / am / was... am... Interesing stuff, port theory! You would expect the big port head to go better but you need the gas flow to fill them plus it's possible to go too far size wise I suppose. For racing it's acceleration that is so important & if your ports are big, great at high speed but take ages to get there then not so good. I was always under the impression that smaller bores had some torque advantage. Careful tuning of intake & exhaust lengths assists. Maybe the rev ability of your very small port head was partly due to other factors? I think there was some discussion in Mr Pike's memoirs on such matters.
One of the Goldie tuners of recent years did some work with smaller & larger bore pipes (& carbs?) for different circuits.
With current crank proposal I would be worried about high revs! I believe some modern motors have gone to longer rods, despite very short strokes, presumably easier piston life also more "dwell" around TDC. Going on current bike power outputs they can't be wrong!
Have been theoretically avoiding spending too much time & money on all my bikes since I first started (not saying that has been successful as a strategy!) hence "off the shed-shelf parts". Time moves on & I really need to get this thing together..
re cutting down pistons, I have an old pair of Omegas that have that amount of scallop to clear flywheels, they were worn out but no issues with the scallops, the lowest point on the scallop just meets the radius at the top of the gudgeon pin boss . I think they had been in a stroker motor, the clearance wasnt needed with my stock stroke crank. When they did run in the very worn state on the stock crank they were a bit "slappy", the rings / bores were pretty worn.
71 Devimead A65 750 56 Norbsa 68 Longstroke A65 Cagiva Raptor 650 MZ TS 250 The poster formerly known as Pod
If you enjoyed the read on the goldies, have a read on the Ariel 650 twins with low performance problems. Long story cut short, the Ariel version of the A10 had everything similar to the BSA variant apart from it had bigger down pipes than the beezer, they fitted A10 pipes and it cured the performance problem... bad news was Ariel had no interest in changing the design and it went no further.
I’ve spent about the last 5/6 years putting into practice things I learned from those memoirs, things other makes have used on competing bikes and took advice from some very knowledgable guys (like John Healy) who put me On the right track.
The forentioned head was one of 4 I tested with that cam, one standard small port which revved to 8k, the first head I modified which pulled like a train but rev limited at 7200, and the mentioned head that started with really small ports ( 10mm high by 27mm at its widest point, I opened the height up to about 15mm and top revs dropped again. With all this I was using the SRM race cam for the A65 with 1.125” radius followers. Bike pulled like a champ... I had changed the cam and modified the head before I found that there was a clearance Issue between the needle roller mains and the crank. Infact I destroyed two combination bearings before changining to a different needle roller and ball race on the drive side... (previously having an outrigger plate on the DS and roller bearing in the normal place) each change there made big gains.
Thanks, I shall seek it out. Gutting to be removing what were briefly perfectly good bearings shortly after fitting them, particularly with an engine that was showing promise. A guy I met on the IOM, ran a Commando (I think - it was years ago) with massive outrigger bearing attachment. No doubt it worked but looked a bit of a lump. Don't know who John Healy is but his name crops up fairly often, - knowns his onions it seems!
Well.... all is not as it seems! It is actually a 112 stroke crank with an old GS BB / late B33 medium rod, so a big motor. I can actually build this now, have everything to hand but the more I've looked into rod / stroke ratios I am well into the danger area of short life pistons at the very least. I only have one - unreplaceable - barrel to fit so if that got wrecked the project is over. I am about to chase up a guy fairly local to me who makes long stroke cranks, that would mean a little smaller motor but healthy R-S ratio. If I go down this path $$$ I might get a BB spec rod made by carillo or similar. You can get the short rods but to my knowledge the medium ones are unavailable.
The BB34 rod is the same length as the ZB long rods, 7 3/8". The "medium" rod I'm guessing you're talking about would be the BB32 6 7/8" rod? Carillo will make any length or dimension you want, it's just a custom order so costs more.
Yes, I have them sorted in my mind as CB short, BB medium ZB long although there is that variation as you say. There was a crossover between ZB & BB engine castings as well to add to the confusion. Cariillo & their stockists do have a "pattern" listed for DBD32/34 but not the medium or long rod. My understanding is that later B33 ran the BB / 6 7/8" rods but have never seen inside one of them. I would have thought there might be a small demand for the medium rods but the BB's don't tend to get raced like the later motors. I've only seen 2 or 3 at race meetings in 25 years or so. There must be a lot of 7 3/8" rods mostly in iron B31,2,3,4 motors. Some will be as much as 70 years old. They're doing pretty well for that kind of age but I don't suppose they get thrashed like Goldies.
Anyway, it would be a custom order rod and not cheap! Essentially I was trying to keep with the original parts I had, but then the barrel is a one-off & I had problems with the bottom end, then I started getting greedy with capacity & headed down the long stroke path. It has been an interesting learning experience but very long winded! Would have been a lot quicker if I just went out & bought a custom crank years ago...
Yes, for the bike it is a bigger bore than standard but not by much as I'm stuck with the barrel I have. That won't go out more than another 1/2 mm. It's all enough to get a bit of a capacity boost with a longer stroke as well.
The ZB34GS rod and the BB34GS rod were the same length. The CB and later DB and DBD rods were the same short rod. The only one different then those was the BB32 rod as mentioned earlier. So there was no Long, medium, short rods for the 500 cc engines. Are we on the same page?
Going back to your original question The area at the sides of the piston under the gudgeon pin holes is irrelevant, the main thrust areas are the front and rear of the skirt. I would have no qualms about cutting the piston to clear the flywheels.
Have a look at This Top left is the piston in a 500 single that produces between 80 and 85 BHP @ 12,500-13,500 RPM